In general, when executing a handover procedure, a UE (user equipment) reports channel state information including signal strength of a serving base station and signal strength of neighboring base stations to the serving base station through a measurement report message. Upon receiving this message, the serving base station determines whether to hand over the UE, based on the received information, and instructs the UE to perform handover. In this case, the serving base station transmits an RRC (radio resource control) connection reconfiguration message to the UE to instruct the UE to perform handover. Upon receiving a message indicating the handover command, the UE immediately performs handover to cut off its wireless connection with the serving base station and attempt a connection to a target base station. The UE then attempts a random access to the target base station and the target base station transmits a random access response as a response to the random access. Next, the target base station determines that the handover is successful, and may start to transmit downlink data.
However, this handover procedure may have disadvantages such as making it difficult for a UE to send a signal strength report in real time to a base station because the radio link is not in a good state and making it difficult for the base station to send a handover command in real time to the UE. As a result, the handover is more likely to be failed, the interruption time of data transmission to the UE during handover increases, and causes an inevitable decline in communication quality during handover.
Also, to increase radio capacity, high-capacity wireless services are being provided to offer a higher-quality radio environment with abundant available resources to a plurality of UEs by miniaturizing service cells. In these services, unlike a general base station (hereinafter referred to as ‘macro base station’), a base station that provides a communication service to UEs existing in a small cell, which is a small communication region such as a office, residence, or building, is referred to as a small base station. Small base stations that serve small cells exist within a macro cell area served by a macro base station.
Under this communication environment, if the UE wants to change cells, it cuts off the connection with the old cell and connects to a new cell to perform handover.
In the way, the UE frequently changes cells under a mobile communication environment with many small cells built therein, handover occurs frequently, and this may lead to a decline in communication quality and significant signaling overhead. Moreover, there is a need to properly establish a method for configuring radio resources for a UE in case small cells are built in a mobile traffic adaptive heterogeneous network which is divided into a control plane and a user plane.